1. Field of the Invention
The present invention relates generally to chronically implantable tissue stimulators and more particularly relates to insulated leads for chronically implantable tissue stimulators.
2. Description of the Prior Art
Tissue stimulators have been used for some time. Much work has been done in the area of stimulation of the myocardium in artificial cardiac pacing. Insulated leads have been developed to conduct the stimulation signal from the implantable pulse generator to myocardial tissue. Certain lead designs such as taught in U.S. Pat. No. 4,154,247 issued to O'Neill have employed a unique shape to ensure stability of electrode placement.
Implantable pulse generators have also been designed to interact with tissue within the neurological system to produce various desired medical effects. The most common application is the use of such a system to control pain. The PISCES.RTM. implantable spinal cord stimulator system available from Medtronic, Inc. of Minneapolis, Minn. is an example of such a stimulation system.
As with the field of myocardial stimulation, neurological stimulation systems must address the problem of proper electrode placement. U.S. Pat. No. 3,646,940 issued to Timm, et al. is an early example of an electrode system which is positioned by the surgeon and then sutured into place. A positioning scheme more expressly designed for the spinal cord is discussed in U.S. Pat. No. 4,285,347 issued to Hess. The Hess technique employs a shaped protrusion at the distal tip to hold the electrode in position.
The PISCES-SIGMA.RTM. neurological electrode also available from Medtronic, Inc. has a sigmoidal shape to fix electrode position. However, this device, as well as the Hess electrode, have the shaped fixation device proximal to the actual electrode. Therefore, it is primarily the distal tip of the insulated lead which becomes fixed in position
A major problem with these prior art leads is that the physician is limited in his ability to acutely and chronically select the site of stimulation within the spinal cord. Currently, this selection is performed at implantation by requiring the surgeon to surgically or percutaneously place the electrode at the exact position to be stimulated.